This reverts commit a3a9664ed2.
Constifying the autogenerated get_instance_private functio makes C++
compilers and GCC with -Wcast-qual warn during compilation of GLib and
projects depending on GLib.
Since using const with GObject instances is not a common coding
practice, it's better to revert than trying to make every sigle GType
function const-safe (and possibly add more compiler warnings in the
process).
https://bugzilla.gnome.org/show_bug.cgi?id=745068
85a23529ab